iStockphoto/Thinkstock(CHICAGO) -- The National Weather Service warned Wednesday that 19 states could experience dangerous weather as a massive line of powerful storms pushes across the Midwest. Parts of Iowa, Illinois and Wisconsin are under tornado warnings.

A number of those storms could converge over a major American city, said ABC meteorologist Ginger Zee.

"It means that everyone's on really high alert. I mean the words 'slight risk' [and] 'moderate risk' don't always give the great gravity to what's going to happen, but tonight a lot of folks -- and we're talking millions and millions, almost 12 million -- are in the high-risk zone, and Chicago making up a lot of that," she said.

Tornadoes have already been reported in eastern Iowa, while severe damaging winds and tennis ball-sized hail reports have come out of Chicago.

Zee warns families in high-risk areas to plan ahead.

"A warning can give you 15 to 30 minutes, but our [storm] watches are out for hours and you should know that you should be within the area that you can get to safety within 10 to 20 minutes," she said. "And that's what we'd like to really emphasize, is it's all about personal responsibility. You have to have a way to get warned. There's a lot of great weather apps out there, and, of course, the biggest one [is] the NOAA Weather Radio that you can have right in your home, battery-operated."

Zee added a special note of caution about how drivers can protect themselves if they are suddenly caught in violent weather.

"If you absolutely cannot get to shelter and say you've got yourself stuck in traffic and you're in your vehicle, pull over to the side of the road, put your flashers on, buckle your seatbelt, duck your head down and cover it with a bag or with a sweatshirt, something that could protect you if the glass broke or flying debris," she said.